The present invention relates to a surface-type light amplifier device usable as a surface light emitting laser etc. when a resonator is disposed outside the device and to a method for the manufacture thereof. The xe2x80x9csurface-typexe2x80x9d light amplifier device referred to herein is a device comprising a light function portion for amplifying and emitting light and a substrate for physically supporting the light amplification function portion, wherein the emitted light rises with a specific angle relative to the surface of the substrate, generally in the direction of intersecting the substrate surface at right angles (in the normal direction).
As for surface-type light amplifier devices of this type, there is the one disclosed in Reference Literature 1: xe2x80x9cElectrically pumped mode-locked vertical-cavity semiconductor lasersxe2x80x9d (W. Jiang, M. Shimizu, R. P. Mirin, T. E. Reynolds and J. E. Bowers, Optics Letters, Vol. 18, No. 22, pp. 1937-1939, 1993). As shown in FIG. 2, the prior art surface-type light amplifier device 30 structurally comprises an n-type GaAs substrate 31 on which a multilayer reflecting mirror of n-type semiconductor 32, an n-type cladding layer 33, an n-type GaAs active layer 34, a p-type cladding layer 35, a p-type AlGaAs layer 37 and a p-type GaAs contact layer 38 are deposited in the order mentioned. The p-type AlGaAs layer 37 and p-type GaAs contact layer 38 are partially cut off into a shape having a shape having a predetermined surface area. An antireflection coating 39 is deposited onto the top surface of the p-type GaAs contact layer. A surface electrode 40 is formed on the p-type cladding layer 35, with an insulating film 36 sandwiched therebetween, so as to surround the cut-off portions and come into contact with the top peripheral part of the p-type GaAs contact layer 38. A substrate electrode 41 is deposited on the bottom surface of the n-type substrate 31.
Injection of carriers into the n-type GaAs active layer 34 is attained by current injection, i.e. by applying voltage between the surface electrode 40 and the substrate electrode 41.
Holes are injected from the surface electrode 40 into the n-type GaAs active layer 34 sequentially via the p-type GaAs contact layer 38, p-type AlGaAs layer 37 and p-type cladding layer 35. Electrons are injected from the substrate electrode 41 into the n-type GaAs active layer 34 sequentially via the n-type GaAs substrate 31, multilayer reflecting mirror of n-type semiconductor 32 and n-type cladding layer 33.
When the prior art device 30 is used as a light amplifier device or particularly as a surface-emitting laser, the associated resonator comprises the multilayer reflecting mirror of n-type semiconductor 32 built in the device and an external reflecting mirror (not shown). Between the external reflecting mirror not shown and the antireflection coating 39 there is generally disposed a lens (not shown). It goes without saying that the antireflection coating 39 is used for reducing the resonator loss and obtaining the light gain. For the same reason, the layers 33 to 35, 37 and 38 are subjected to treatments such as for suppressing the impurity concentration etc. to a low degree so that the optical absorption loss can be lowered.
FIG. 3 shows another prior art surface-type light amplifier device 50. This device is disclosed in Reference Literature 2: xe2x80x9cHigh single-transverse-mode output from external-cavity surface-emitting laser diodexe2x80x9d (M. A. Hadley, G. C. Wilson, K. Y. Lau and J. S. Smith, Appl. Phys. Lett., Vol. 63, No. 12, pp. 1607-1609, 1993) and comprises a GaAs substrate 51 not of n-type but of p-type, on which a multilayer reflecting mirror 52 of p-type semiconductor, a p-type multi-quantum well active region 53 and a multilayer reflecting mirror 55 of n-type semiconductor are deposited in the order mentioned. Voltage is applied between a substrate electrode 57 deposited on the bottom surface of the substrate 51 and a bonding pad 56 disposed on an insulating film 54 and brought into contact with the top peripheral surface of the multilayer reflecting mirror of n-type semiconductor 55 to inject an electric current (carriers) into the multi-quantum well active region 53, thereby obtaining excited light. Holes are injected from the side of the substrate electrode 57 into the multi-quantum well active layer 53 via the p-type GaAs substrate 51 and the multilayer reflecting mirror of p-type semiconductor 52, whereas electrons are injected thereinto from the opposite side, i.e. from the bonding pad 56, via the multilayer reflecting mirror of n-type semiconductor 55.
This device 50 is, by nature, not a device for an external resonator. However, in the case that a resonator is composed only of the multilayer reflecting mirror of n-type semiconductor 55 and the multilayer reflecting mirror of p-type semiconductor 52 embedded in the device, it inevitably poses a substantial problem that the transverse mode does not become a single lobe when the diameter of the device is made large. In order to solve the problem it is necessary to provide an external reflecting mirror not shown. Single-lobe beams can be obtained by deliberately lowering the reflecttivity of the multilayer reflecting mirror of n-type semiconductor 55, then providing a suitable reflecting mirror outside the device on the side of the multilayer reflecting mirror of n-type semiconductor 55, and adjusting the position of a lens disposed in an optical path toward the external reflecting mirror, for example. In any event, the resonator has a composite construction comprising a first resonator composed of the multilayer reflecting mirror of p-type semiconductor 52 and the multilayer reflecting mirror of n-type semiconductor 55 which are provided in the device and a second resonator composed of the multilayer reflecting mirror of p-type semiconductor 52 and the external reflecting mirror.
In the device 30 shown in FIG. 2, however, it is particularly difficult to obtain laser beams having a large diameter. This is because, if the effective area of the n-type GaAs active layer 34, i.e. the area coated with the antireflection coating 39 and actually contributing to oscillation, is made large for enlarging the device diameter, it will become impossible to uniformly inject holes into that area. This results solely from the fact that each of the p-type semiconductor layers 38, 37 and 35 has high electrical resistance. In order to inject holes into the neighborhood of the center of the effective area of the n-type GaAs active layer, it is necessary to cause the holes first to flow through the p-type semiconductor layers 38, 37 and 35 in the in-plane direction from the surface electrode 40 in contact with the peripheral edge of the antireflection coating 39 and then to be injected into the center of the n-type GaAs active layer 34. In the actual course of operation, however, this cannot be attained because the majority of holes are injected into the peripheral edge of the p-type GaAs contact layer 38 from the surface electrode 40 and then advance straightforward without being well spread laterally and reach the n-type GaAs active layer 34.
In order to actually secure the state of uniform hole injection into the n-type GaAs active layer 34 in the conventional device 30 fabricated in accordance with such structural principle, it is required to reduce the diameter of the effective area of the n-type GaAs active layer 34 to not more than tens of xcexcm. That is to say, when a large output power is required, it is necessary to adopt a method of arraying a plurality of devices, resulting in sacrifices of singleness and uniformity of optical beams.
In the conventional device 50 shown in FIG. 3, however, since holes can be injected from the substrate electrode 57 in surface contact with the back surface of the p-type GaAs substrate 51, the uniformity in the in-plane distribution of holes injected into the p-type multi-quantum well active region 53 will be satisfied. However, the serious problem is that the device has a composite resonator structure which cannot constitute a pure external resonator type surface light emitting laser and since the multilayer reflecting mirror of n-type semiconductor 55 incorporated in the device generally has a reflectance of not less than about 80%, the device is not suitable as a surface-type light amplifier device. In addition, due to the composite resonator structure, light pulses cannot be generated in the mode-locking operation.
Furthermore, since the multilayer reflecting mirror of n-type semiconductor 55 having a resistance lower than that of a p-type one is used for the sake of electron flow in the in-plane direction, the structure is designed for injecting electrons into the neighborhood of the center of the multi-quantum well active region 53. However, if the diameter of the multi-quantum well active area 53 is set larger, the electrical resistance of the multilayer reflecting mirror of n-type semiconductor 55 cannot be ignored and unevenness in the current injection is induced. That is to say, for uniform current injection, the upper limit of the effective area of the multi-quantum well active area 53 is about 100 xcexc in diameter though it is larger than that of the conventional device 30 shown in FIG. 2. In particular, it is impossible to control the injection of holes into the active layer because the p-type electrode is the substrate electrode and an electric current is injected through the substrate.
The present invention has been proposed in view of the problems mentioned above, and its object is to provide a surface-type light amplifier device having at least a light amplification section including a structure of an active layer sandwiched between p-type and n-type cladding layers and emitting a light beam in the direction rising with a specific angle (generally, 90xc2x0 as stated above) relative to the surface of a support substrate, wherein the amplification of a single and uniform light beam or, if required, a large-diameter light beam, and the laser oscillation can be attained.
The inventor believes that, in the final analysis, the various drawbacks of the conventional devices 30 and 50 shown in FIGS. 2 and 3 result from the presence per se of the n-type substrate 31 or the p-type substrate 51 forming the light amplification section contributing to the light amplification, namely, the multilayer structure including the semiconductor layers 32-35 and 37-38 in the device 30 shown in FIG. 2 or the multilayer structure including the semiconductor layers 52, 53 and 55 in the device 50 shown in FIG. 3.
It goes without saying that the substrate 31 or 51 is indispensable to the formation of the light amplification section and important even after the section has been formed as a support for securing the physical strength of the device. Insofar as the light amplifying function is concerned, however, the substrates 31 and 51 are rather unnecessary or obstructive. Since the substrates 31 and 51 generally have a large thickness of up to hundreds of xcexcm, when a compound semiconductor substrate such as a GaAs substrate is employed, loss in passing amplified light therethrough is very large.
For this reason, both the conventional devices 30 and 50 shown in FIGS. 2 and 3 have a construction such that amplified light is not passed through the support substrates 31 and 51. This is the same in other conventional devices not touched upon here. In other words, various changes in construction for improving the characteristics of devices have heretofore had to be made on the major premise that light should not be passed through a substrate. This has brought about various restrictions. In the case of the conventional device 30 shown in FIG. 2, for example, since the light emitted from the light amplification portion has to be emitted from the side of the p-type semiconductor layers 35, 37 and 38 opposite the side at which the n-type GaAs substrate 51 is present, this light emitting surface cannot be covered by the electrode. As a result, an electric current has to be applied only through the peripheral edge of the p-type AlGaAs layer 37 to the the p-type cladding layer and then to the n-type GaAs active layer 34 as described above, thereby inducing the aforementioned uneven hole injection and difficulty in achieving a large device diameter.
In the case of the conventional device 50 shown in FIG. 3, the p-type GaAs substrate 51 is used in place of the n-type GaAs substrate, with the result that there is an advantage that the multilayer reflecting mirror of n-type semiconductor 55 can be disposed on the side opposite the side at which the substrate is present to achieve low resistance, but there are restrictions, such as requiring a composite resonance structure, resulting in the different drawbacks as described above.
In view of the above, the present inventor, exploding the well-established concept, has conceived the idea of removing a base substrate used for fabricating a light amplification section after the fabrication of the light amplification section. However, since the light amplification section is an extremely thin structure, such mere removal of the base substrate would decrease the strength of the light amplification section resulting in physical distortion producing optical distortion, and would not allow the light amplification section to be put into practical use. Therefore, the present invention provides a structure having the light amplification section attached to a transparent support substrate that is separate from the base substrate used in fabricating the light amplification section and exhibits a low loss when a light beam passes therethrough.
With this device structure, a light beam amplified at the light amplification section can be passed through the transparent substrate. This means that there arises a degree of freedom in structural design. For example, an electrode through which holes are injected into a p-type semiconductor layer with relatively high resistance can be made large. Furthermore, even when a plurality of divided electrodes are formed and a light beam is prevented from being emitted from the side of these electrodes as in the specific embodiment of the present invention which will be described later, various improvements can be realized by enabling a light beam to be emitted via the transparent substrate provided on the opposite side of the electrodes with the active layer of the light amplification section therebetween.
The present invention also provides a surface-type light amplifier device, as a preferred unsophisticated embodiment satisfying the above fundamental conditions, having a light amplification section attached to a transparent substrate on the side on which an n-type semiconductor layer is present and having a plurality of divided electrodes provided on the side across an active layer opposite the n-type semiconductor layer for injecting holes into a p-type semiconductor layer.
In this surface-type light amplifier device, it is possible to uniformly inject holes into the p-type semiconductor layer having higher resistance than an n-type one. In addition, since the in-plane distribution of the carriers in the active layer can be controlled by controlling the amount of an electric current applied to the divided electrodes, it is possible to control the in-plane distribution to conform to the light intensity distribution in the fundamental mode having a single lobe.
The present invention provides, as a more concrete unsophisticated embodiment, a surface-type light amplifier device wherein an active layer in a light amplification section is sandwiched between an n-type semiconductor cladding layer that is an n-type semiconductor layer and a p-type semiconductor multilayer reflecting mirror that is a p-type semiconductor layer; the light amplification section is attached to a transparent substrate on the side of the n-type semiconductor cladding layer; a plurality of divided electrodes attain electric continuity through a p-type cap layer provided on the p-type semiconductor multilayer reflecting mirror; and an electrode forming electrical continuity relative to the n-type semiconductor cladding layer is connected to a wiring conductor provided on the transparent substrate.
The present invention further provides a method for fabricating a surface-type light amplifier device, which includes the steps of forming a light amplification section on a structural substrate for the formation of that section, attaching a different transparent substrate to the exposed surface of the light amplification section, and removing the structural substrate.
As an unsophisticated embodiment of the aforementioned fabrication method, the present invention provides a fabrication method comprising the steps of successively forming a p-type semiconductor layer, an active layer and an n-type semiconductor layer in the order mentioned on the structural substrate, attaching the transparent substrate to the exposed surface of the n-type semiconductor layer, and forming a plurality of divided electrodes on the surface of the p-type semiconductor layer exposed after the removal of the structural substrate, thereby attaining electrical continuity relative to the p-type semiconductor layer.